Polyamidoimides have already frequently been described in the literature as resin components of wire enamels. In this context, the polyamidoimides described in DE 2 203 153, EP 0 315 925 or DE 1 720 356 may be mentioned as examples.
However, the solutions of such polyamidoimide resins in dipolar aprotic solvents, such as dimethylformamide, dimethylacetamide, N-methylpyrrolidone, tetramethylurea or N,N′-dimethylethyleneurea, diluted with aromatic solvents, such as xylene and solvent naphtha, have a low storage stability. The literature discloses in particular the crystallization and gelling in combination with an increase in viscosity as disadvantages of such resins. In particular, symmetrical systems having amido-imido-imido-amido groups tend to crystallize. For this reason, the incorporation of further monomers, for example aliphatic and aromatic dicarboxylic acids, as co-components was described as a possible solution to the crystallization problem in some publications, such as, for example, in DE 2 203 153 and EP 0 315 925.
A further possibility for improving the storage stability was mentioned in DE 2 203 153 and consists in the use of mixtures of aromatic diamines, such as, for example, 2,4′- and 4,4′-diaminodiphenyl oxide (example 1 in DE 2 203 153) or of mixtures of 4,4′-diaminodiphenylmethane with 4,4′-diamino-3,3′-dimethyldiphenylmethane.
Mixtures of the isocyanate components, such as 4,4′-diisocyanatodiphenylmethane, together with industrial toluene diisocyanate were also used (example 6 of DE 2 203 153).
A further method for improving the storage stability and stability during enameling was described in DE 4 004 462. There, the enamels are protected from gelling by drying the polyamidoimide solution using a zeolite-based drying agent. However, the zeolite particles used in this process step have to be filtered off in order to avoid contamination of the enamel surface. This method is therefore another method which has disadvantages since the additional filtration step requires careful monitoring in order to avoid the deposition of particles on the coated wire.
EP 0 315 925 discloses that the molecular weight of polyamidoimides can be adjusted by adding monocarboxylic acids, such as formic acid, acetic acid or aromatic acids, such as benzoic acid, which are preferably intended to react with the active centers of the polyamidoimides.
Primary alcohols are described in the literature as further components for stabilizing polyamidoimides in order to prevent secondary reactions. With the processes disclosed in the literature, however, it is not possible to prepare storage-stable polyamidoimides, since low concentrations of active centers always remain in the enamels.
All abovementioned polyamidoimide coating materials and resins are unstable under storage and application conditions, in particular in the presence of relatively high humidity and temperature. It was therefore the object of the present invention to convert as far as possible all reactive centers of the polyamidoimides into groups which prevent crosslinking of the linear polymer in the presence of water or humid air. Suitable reactive centers in polyamidoimides, in addition to free isocyanates, are in particular isoimides, allophanates, biurets, uretdiones, carboxylic anhydrides and blocked isocyanates. The prevention of crosslinking is all the more important since even a few crosslinkings of the monomer units of the polymer can result in a considerable increase in the viscosity of the polymer solution.
The object of the present invention was therefore to provide an economical process for the preparation of storage-stable polyamidoimide resins and coating materials, which does not have the abovementioned disadvantages.